1. Field of the Invention
The present invention relates to a method used in a communication device in a wireless communication system, and more particularly, to a method of monitoring a call state over a single radio voice call continuity.
2. Description of the Prior Art
Internet-Protocol (IP) Multimedia Subsystem, hereafter called IMS, is a set of specifications that describes the Next Generation Networking (NGN) architecture for implementing IP based telephony and multimedia services. IMS defines a complete architecture and framework, which enables a convergence of voice, video, data and mobile network technology over an IP-based infrastructure. Session Initiated Protocol (SIP), Session Description Protocol (SDP), and Real-Time Transport Protocol (RTP) defined by the Internet Engineering Task Force (IETF), are chosen as main protocols for IMS. IMS fills a gap between the two most successful communication paradigms, cellular and Internet technology, and thereby enables users to surf Webs, play online games or join a videoconference through 3G handheld devices, i.e. user equipments (UEs). In short, IMS provides cellular access to all the services that the Internet provides. However, Long Term Evolution (LTE)/High Speed Packet Access (HSPA) only provide PS (packet-switched) services. Voice over IP (VoIP) is the only solution to support voice calls in an LTE/HSPA network.
A Single Radio Voice Call Continuity (SRVCC) refers to a voice call continuity between IMS over PS (packet-switched) access and CS (circuit-switched) access for calls which are anchored in IMS when a user equipment (UE) is capable of transmitting/receiving on only one of PS and CS access networks at a given time. On the other hand, a Single Radio Video Call Continuity (vSRVCC) refers to a video call continuity from an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) to a UTRAN-CS for calls which are anchored in IMS when the UE is capable of transmitting/receiving on only one of the E-UTRAN and UTRAN-CS at a given time.
Please refer to FIG. 1, which is a schematic diagram of a wireless communication system 10. The wireless communication system 10 may include a PS network, a CS network and a UE. The PS network may be an LTE or HSPA network, and may include a plurality of LTE/HSPA base stations to provide IMS calls. The CS network may be a Mobile communications (GSM), Universal Mobile Telecommunications System (UMTS), or Code Division Multiple Access 2000 (CDMA2000) network, and may include a plurality of GSM/UMTS/CDMA2000 base stations to provide CS calls. The UE may be a device such as a mobile phone, a computer system, etc. Besides, the network and the UE can be seen as a transmitter or receiver according to transmission direction, e.g., for uplink (UL), the UE is the transmitter and the network is the receiver, and for downlink (DL), the network is the transmitter and the UE is the receiver.
The UE may establish an IMS call with the PS network or establish a CS call with the CS network, and demonstrate a call state of the IMS/CS call on a user interface (not shown in FIG. 1) to its user. The PS network may request the UE to perform SRVCC handover for the IMS call from the PS network to the CS network by sending an SRVCC handover command, and the CS network may request the UE to perform SRVCC handover for the CS call from the CS network to the PS network by sending an SRVCC handover command.
However, the UE may physically be composed of two operating environments, one is called application side for running an operating system, providing the user interface (UI) and showing the call state on the UI, and the other is called modem side to perform wireless communication based on 3GPP/3GPP2 stacks. Besides, the IMS call may be implemented similar to some VoIP applications (e.g. Skype™), such that an IMS operation based on RTP, SIP, UDP, and IP protocol stacks, may be performed by the application side. Of course, the IMS operation may be performed by the modem side.
Applicant notices that the application side may have a difficulty to monitor a call state of a call over an SRVCC handover. After an SRVCC handover, assume that an IMS call is handed over to a CS call. A call state of the CS call may be initialized according to 3GPP/3GPP2 specification in the modem side. However, the application side does not get the call sate of the CS call since the call sate of the CS call is maintained in the modem side (e.g. different processors and different storages). Besides, the application side may keep running IMS protocol stack and encoding voice packets, which consumes power since the application side cannot enter a sleep mode (e.g. application processor cannot run in a lower clock). Similar issue also happens in SRVCC handover from the CS network to the PS network.
Therefore, there is a need to solve the above problems.